1. Field of the Invention
The field of the invention is that of X-ray generator tubes. The invention relates more specifically to the arrangement of the emitting surfaces which are the source of the X-ray radiation.
2. Description of the Prior Art
The principle of operation of an X-ray generator tube 10 is set out in FIG. 1. It mainly comprises a vacuum chamber 6 comprising, at one of its ends, a cathode unit 4 borne by an insulator 3 and, at the other end, an anode unit 2. The anode unit 2 comprises a target carrier assembly 1 comprising a flat metal surface known as the target 9 positioned facing the cathode unit. The electron beam 7 originating from the cathode is accelerated under the action of very high electrical voltages in excess of 10 kVolts and strikes the target 9 in a focusing region O where the electrons lose their kinetic energy. This results in a significant release of heat and in an emission 8 of X-ray radiation (symbolized by the arrows in FIG. 1). The X-ray radiation passes through the wall of the anode unit at favored locations 5 known as windows.
The release of heat causes very intense localized heating at the target. In the case of tubes operating at high power, the rise in temperature of the target is such that it could cause the target to become destroyed by melting. Hence, in such cases, the release of heat is removed by a cooling circuit 60 passing through the target carrier 1 under the target 9.
In order to optimize the distribution of the X-ray radiation in space in terms of direction and in terms of intensity, the target 9 is inclined by an angle α with respect to the mean direction of the electron beam 7.
The production of a target carrier assembly therefore is subject to two main constraints: on the one hand, the angle of inclination α needs to be suited to the use and, on the other hand, the cooling circuit needs to allow sufficient removal of heat energy due to the impact of the electron beam.
In known X-ray radiation tubes, the target carrier assembly generally has the shape of a stepped cylinder as depicted in FIGS. 2, 3 and 4. The axis of this cylinder is parallel to the direction of the electron beam. A truncated face of the cylinder inclined by an angle α constitutes the target subjected to the action of the beam.
When the power is low, a cooling circuit is not needed. In this case, which is illustrated in FIG. 2, the target carrier assembly is connected to the anode unit so that the heat energy is transmitted first of all to the periphery of the anode unit by conduction through the various metal parts of the target carrier assembly and of the anode unit (internal white arrows in FIG. 2) then removed to the outside by convection (external white arrows in FIG. 2).
When the emitted power is higher, the above arrangement will no longer suffice. In such cases, a circulation duct for cooling fluid which may, for example, be water or oil, is needed in order to remove the heat energy from the target. This fluid is let in and out in the part of the target carrier assembly at the opposite end from the target. FIG. 3 illustrates a first embodiment of the cooling duct positioned inside the target carrier assembly. It comprises a single tube 60 passing under the surface of the target and which follows the lines of said surface as best it can. FIG. 4 illustrates a second embodiment of this duct, of a coaxial type. It comprises an inlet tube 60 lying along the axis of the cylinder of the target carrier, an internal cavity 61 following the lines of the interior of the target carrier as best it can, and an outlet tube 62 connected to the internal cavity. This arrangement is able to optimize the area for heat exchange between the cooling fluid and the target carrier assembly.
However, these various types of cooling circuit have disadvantages. In particular, these ducts have elbows which lead to changes in direction for the fluid. These changes in direction generate, at the surfaces for heat exchange with the target carrier assembly, regions in which the velocity of the fluid is practically zero and in which the heat exchanges are therefore very low. In addition, these changes in direction induce pressure drops which may prove prohibitive when the fluid flow rate needs to be increased in order to improve the heat dissipation capabilities.
When an electron beam strikes the surface of the target at an angle of incidence α corresponding to the angle of inclination of the target, the X-ray radiation is emitted in all directions in space as indicated in FIG. 5. The emission intensity profile is dependent on the angle θ made by the direction of the radiation with respect to the normal N to the surface of the target (the boundary depicted in dotted line in FIG. 5). This profile exhibits a maximum for zero θ and tends toward 0 as θ tends toward 90 degrees. Not all of the X-ray radiation emitted can be used, and only some is collected through a transmission window which defines a limited solid emission angle. This window is necessarily situated outside the path of the electron beam. If a significant proportion of the emitted radiation is to be recovered, the angle of inclination α has then to be sufficiently great.
However, the angle of inclination also governs the geometric resolution of the X-ray emission source as illustrated in FIGS. 6 and 7. An electron beam 7 of circular cross section with diameter Ø, a cross section also known as the fineness, impinges on a target inclined by an angle α with respect to the direction of incidence. This beam will generate X-ray radiation. In a given emission direction, the X-ray radiation, passing through a very small-diameter diaphragm 11, then has a divergence β. This divergence β is proportional to the angle α as shown in FIGS. 6 and 7. This divergence β governs the resolution of the X-ray generator tube and the sharpness of the perceived images. Indeed, if a screen 12 is placed in the path of the X-ray radiation, the image of the diaphragm is no longer practically an isolated spot but has a certain dimension directly proportional to the divergence β. In consequence, in order to obtain small-sized images, that is to say high resolutions, it is necessary to reduce the angle of inclination α.
The angle of inclination α is, of necessity, the result of a compromise between, on the one hand, the energy of the X-ray radiation and, on the other hand, the resolution of the tube. Depending on the application, tube designers therefore have, for the same tube configuration, to provide different versions of target carrier assembly in which the angles of inclination of the target vary. Designing, producing and managing these different variants leads to additional costs and longer time scales which may be great, given the complexity of the part and the materials used.